World of Intricate Muscles Revealed Inside Velvet Worm's Wee Leg

By Mindy Weisberger, Senior Writer |
December 12, 2017 07:04am ET

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When viewed under powerful magnification, tiny organisms whose smallest body parts are too minute to be seen with the naked eye are revealed in breathtaking complexity. And now, scientists have developed a method for peering inside structures that measure fractions of a millimeter, and can even image them in three dimensions — a feat that previously had been impossible.

For decades, X-ray computer tomography (CT) scanning has enabled scientists to noninvasively examine the insides of organisms and objects, and model them in 3D. But the technology only worked on subjects that were larger than 500 nanometers (a nanometer is 1-billionth of a meter, or 400-billionths of an inch).

Recently, scientists developed a tabletop Nano-CT system capable of capturing images in 3D at an unprecedentedly small scale — 100 nanometers. Its limits were recently tested on a velvet worm’s minuscule legs, which measure a mere 0.02 inches (0.4 millimeters) long, and this novel technology successfully visualized individual muscle fibers inside the worm's leg, the researchers reported in a new study. [Images: Tiny Life Revealed in Stunning Microscope Photos]

When an object is CT-scanned, multiple X-ray images are taken from many angles, creating cross-sectional views of the object's internal structure. Using computer processing, these individual image "slices" are then combined to rebuild the interior of the image in 3D, according to the Mayo Clinic.

Nano-CT uses nanotubes to tightly focus X-rays and visualize much smaller objects at higher resolution than had been possible with CT scans until now. As the process creates a digital 3D model of the object from a single scan, it is cheaper and less time-consuming to use than other high-resolution imaging methods that can only capture 2D images on a single plane, such as scanning electron microscopy (SEM) and confocal laser scanning microscopy (CLSM), the researchers explained in the study.

Nano-CT images of a velvet worm leg: The image on the left shows the surface of the leg, while the image on the right reveals muscle fibers inside the tissue.

Credit: Muller/Pfeiffer/TUM/Reproduced with permission from PNAS

The scientists tested the system by looking inside the legs of the tiny velvet worms —soft-bodied animals that resemble worms with multiple sets of limbs. They are part of the group panarthropoda, which includes arthropods and tardigrades. The scans revealed that the worms' feet contained circular muscles, which had been hinted at in prior studies but were not meticulously described, the scientists reported.

"Our data confirm the existence of these muscles and reveal details of their position, arrangement and size," the researchers wrote. And the characteristics of the muscles suggest that they are used to extend claws in the feet, but the shape and function of most of the velvet worm's muscles are still unknown, according to the study.

Velvet worms are an ancient lineage that has changed little in 500 million years, and their closest relationships on the tree of life are still being debated, the scientists wrote in the study. Internal examination of their delicate limb structures could offer scientists new insights into the animals' locomotion, and may help researchers puzzle out how segmented limbs in arthropods evolved, study co-author Georg Mayer, head of the Department of Zoology at the University of Kassel, said in a statement.

There could also be biomedical applications for this technology, according to Franz Pfeiffer, a professor of biomedical physics at the Technical University of Munich (TUM) and a fellow at the TUM Institute for Advanced Study (TUM-IAS).

"We will be able to examine tissue samples to clarify whether or not a tumor is malignant," Pfeiffer explained in the statement.

"A non-destructive and three-dimensional image of the tissue with a resolution like that of the nano-CT can also provide new insights into the microscopic development of widespread illnesses such as cancer," Pfeiffer said.

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Mindy Weisberger is a senior writer for Live Science covering general science topics, especially those relating to brains, bodies, and behaviors in humans and other animals — living and extinct. Mindy studied filmmaking at Columbia University; her videos about dinosaurs, biodiversity, human origins, evolution, and astrophysics appear in the American Museum of Natural History, on YouTube, and in museums and science centers worldwide. Follow Mindy on Twitter.